Cemented carbide, also called hard-metal, is class of hard material comprising a hard phase of metal carbides and/or carbo-nitrides, the metal being selected from groups IVa to VIa of the periodic table and a metallic alloy binder comprising one or more iron-group metals. Hard-metals are produced by a powder metallurgy method typically including the steps of milling, mixing, pressing and liquid-phase sintering. The sintering temperatures of the most commonly used WC—Co hard-metals are usually above the melting point of a eutectic temperature, which is in the range of about 1300 deg. C. to 1320 deg. C. The sintering temperatures used for another class of hard-metals called cermets and comprising TiC or TiCN with a Ni—Mo-based binder, are above the melting point in the Ti—C—Ni—Mo system of roughly 1280 deg. C. Typically the sintering temperatures for hard-metals are above 1350 deg. C., which allows the formation of a large fraction of liquid phase during sintering in order to promote full density of the sintered product.
The term “wear part” is understood to mean a part or component that is subjected, or intended to be subjected to wearing stress in application. There are various kinds of wearing stress to which wear parts may typically be subjected such as abrasion, erosion, corrosion and other forms of chemical wear. Wear parts may comprise any of a wide variety of materials, depending on the nature and intensity of wear that the wear part is expected to endure and constraints of cost, size and mass. For example, cemented tungsten carbide is highly resistant to abrasion but due to its high density and cost is typically used only as the primary constituent of relatively small parts, such as drill bit inserts, chisels, cutting tips and the like. Larger wear parts may be used in excavation, drill bit bodies, hoppers and carriers of abrasive materials and are typically made of hard steels which are much more economical than cemented carbides in certain applications.
US Patent Publication No. 2007/0092727 teaches a wear part comprising diamond grains, a carbide phase such as tungsten carbide and a metallic alloy with liquidus temperature less than 1,400 deg. C. and preferably less than 1,200 deg. C. Two methods are taught for making the wear parts. In the first method an intermediate article comprising diamond grains is contacted with a source of both a selected infiltrant first alloy and a selected second alloy, the temperature of the source and intermediate article is raised to above the liquidus of the infiltrant alloy, causing the latter to infiltrate into the pores of the intermediate article. Carbides are formed when components of the second alloy react with the diamond of the intermediate article. In the second method, which is more suitable for making larger wear parts, an intermediate material comprising diamond grains and an alloy selected from the first group and an alloy from the second group is subjected to hot pressing at a temperature lower than 1,200 degrees Centigrade (deg. C.). No infiltration is required in the second method.
Stainless steel alloys developed for the nuclear industry are taught in U.S. Pat. No. 5,660,939 and GB Patent No. 2,167,088, for example, and comprise chromium, nickel, silicon and carbon, but positively do not contain cobalt, which is generally unsuitable for use in a radio-active environment. These alloys are both hard and corrosion resistant.
Materials comprising uncoated diamond grains dispersed within in a hard-metal matrix are disclosed in a number of patents, for example U.S. Pat. No. 1,996,598, GB Patent No. 611,860, German Patent No., 531,077 and Swedish Patent No. 192,637.
The use of diamond grains coated with a layer of a refractory metal carbide, nitride, oxide, boride, suicide or combination thereof as components of hard-metals is described in U.S. Pat. No. 5,723,177. These coatings on diamond grains are believed to suppress or retard graphitisation of the diamond during sintering. However, the above mentioned U.S. patent discloses only conventional hard-metals with relatively high liquidus temperatures at which graphitisation or other degradation of the diamond is accelerated or promoted. For this reason, the diamond-containing hard-metal has to be sintered without the formation of a liquid phase, i.e. at temperatures lower than about 1300 deg. C. Each article comprising the hard-metal as described above has to be hot-pressed individually by means of an axial press in order to obtain a sufficiently high density, in contrast to the more economical conventional batch method of sintering hard-metal articles in large furnaces. Consequently, articles produced according to the teaching of this patent have relatively high production costs. An additional disadvantage of sintering without a liquid phase is that the optimal microstructure and complete elimination of residual porosity within the material cannot be obtained.
Another patent describing a hard-metal comprising coated diamond grains is Japanese Patent No. 2001040446. This reference discloses a hard-metal with a binder comprising Fe-group metals and again, it is taught that the sintering temperatures should be relatively low (nearly 1300 deg. C.) in order to prevent full melting of the binder phase. As a result, the optimal microstructure and full density of the material cannot be obtained.
There are a number of references disclosing diamond-containing hard-metals characterised by a low temperature liquid phase formation. U.S. Patent Publication No. US 2007/0092727 describes a diamond-containing hard-metal comprising a carbide phase and a binder phase comprising a metallic or inter-metallic alloy with a liquidus temperature of less than 1400 deg. C., preferably 1200 deg. C. The diamond grains are not coated so that even at relatively low sintering temperatures the diamond grains, being in contact with the liquid binder containing Fe-group metals, would tend to graphitise.
PCT Patent Application No. PCT/JP2006/301033 describes a diamond-containing hard-metal, the binder of which comprises 0.01 to 2.0 wt % of phosphorus to reduce the temperature of liquid phase formation. A disadvantage of this hard-metal is that, even at high phosphorus content, the binder phase is only partially molten, which tends to result in some residual porosity.
There is thus a need to provide improved metallurgical formulations, particularly hard metals, that result in reduced degradation of diamond or other ultra-hard abrasives such as CBN grains where incorporated. It is desirable that the hard-metal be produced at pressures equal to or lower than atmospheric pressure without using hot-pressing, which would allow the mass-production at low costs.